The book ``The Craft of Prolog'' by Richard O'Keefe is an invaluable source for writing efficient Prolog programs. Many of the techniques and tricks presented are also applicable to writing efficient ECLiPSe programs. The underlying maxim ``elegance is not optional'' summarizes the spirit of declarative programming. As the book is based on a standard Edinburgh style Prolog, it does not handle issues of module structure and in-line documentation, nor other ECLiPSe specific features.
The ECLiPSe documentation contains most of the information provided in this tutorial, but presents it in a very different way. Invariably, it describes all features of the system, many of which are only required for quite specific (if important) tasks like developing new constraint engines inside the system. It can be difficult to find which parts of the documentation contains important hints to solve a particular problem. On the other hand, it will be useful to look up each feature in the user manual and/or the reference manual as they occur in the tutorial.
The software engineering techniques used in this tutorial are mainly adaptations of OO-development methodologies. This field is far too wide to give specific references here, but the excellent Object Technology Series (http://www.awl.com/cseng/otseries) is a good starting point.